Read the light. Not the screen.
Six chapters. Sunny 16, the exposure triangle, the three tricky scenes that catch people out, metering modes, how much each film forgives you, and what happens when the emulsion stops listening. Written for anyone shooting real film on a modern phone.
One rule, memorised once, carried forever.
Sunny 16 is the oldest exposure rule in photography and it is still the most reliable starting point anyone has invented. On a bright, cloud-free day in unambiguous sunlight, set the aperture to f/16 and set the shutter speed to 1 divided by the film's ISO. That is a correct exposure. No meter needed. No app needed. Just the numbers on the box of film and a sunny patch of ground.
Portra 400 at f/16 asks for 1/400s. The closest marked shutter value on most vintage cameras is 1/500, which is close enough to be invisible on colour neg. Tri-X 400 at f/16 also wants 1/400s. Velvia 50 at f/16 wants 1/50s. The ISO rating on the film is the shutter speed when the aperture is f/16 and the sun is out. That is the whole trick.
Every other lighting condition, hazy sun, open shade, overcast, deep shade, is either a stop or a fraction of a stop off that baseline. Once you feel Sunny 16 in your body the numbers stop being numbers. They become stops from the baseline, and after a summer of shooting you can meter a whole roll in your head before the meter finishes calibrating.
This is also the anchor stop. uses. Point the phone at a Sunny 16 reference scene, tap calibrate once, and every reading from then on is adjusted for the drift in your specific iPhone's light sensor. No meter app gets this right out of the box, because every sensor drifts slightly differently from the next.
Once you feel Sunny 16 in your body, the numbers stop being numbers. They become stops from the baseline.
Three dials. One result. Trade between them.
Every correct exposure is a conversation between three things. Change one dial and compensate by the same amount on one of the other two. That is the only rule and every photograph you have ever liked obeys it.
If the meter reads f/8 at 1/250s on Portra 400 and you want to shoot wide open at f/2 to throw the background out of focus, you have opened the aperture by four full stops. To keep the exposure correct you give four stops back on shutter speed. f/2 at 1/4000s. Same exposure, radically different picture. stop. does this arithmetic for you the moment you change one dial.
Aperture. f-stop.
How wide the lens opens. Wider (f/1.4, f/2) lets in more light and collapses depth of field so the subject pops off the background. Narrower (f/16, f/22) lets in less light and keeps everything from foreground to horizon acceptably sharp. Each full stop halves or doubles the light. The number gets bigger as the hole gets smaller, which is the counterintuitive part most beginners snag on.
Shutter speed.
1/1000s freezes a runner mid-stride and lets in almost nothing. 1/60s is the slowest you can realistically handhold a 50mm lens without camera shake. 1s will capture a long exposure of car light trails and needs a tripod. Each step up the shutter ladder (1/500 to 1/250 to 1/125) doubles the light hitting the film.
ISO. The film itself.
You pick ISO when you pick your stock. ISO 50 is slow, needs lots of light, rewards you with fine grain and long tonal latitude. ISO 800 shoots in dim interiors and gives you pronounced grain as a feature. You can push or pull one or two stops when you develop, but the box speed is the honest starting point.
Overcast. The sky is a giant softbox and the meter wants to read it as bright because so much light is bouncing around. Your subject is actually dimmer than the frame looks. Open up one full stop from Sunny 16. On Portra 400 that lands at f/8 at 1/500s, or f/11 at 1/250s if you want more depth of field.
Open shade on a sunny day. Two to three stops under Sunny 16. The sky is still lighting the subject, just indirectly through diffusion. Portra 400 comes in at f/8 to f/5.6 at 1/400s. Useful because shaded portraits retain the blue sky quality in the catchlights without the harsh overhead sun carving shadows under the nose.
Backlight. This is the one that burns beginners. A meter in matrix or centre mode averages the bright sky behind your subject with the dim face in front of you, and it hands you an exposure that turns the face into a silhouette. If you want to keep the glow, switch to spot metering, point the spot at the subject's skin, and overexpose by two full stops from whatever the spot reads. A face in backlight on Tri-X 400 usually needs f/4 at 1/250s instead of the matrix reading's f/8 at 1/250s.
UK-specific note. A huge amount of daylight in the UK is soft high-overcast, not true Sunny 16. Even on a clear winter day the sun angle is shallow enough that Sunny 16 overexposes by roughly half a stop. Calibrate stop. on a clear spring or summer day if you can. A July calibration will carry you accurately through the whole year.
A meter never knows what you're looking at. You do.
Matrix. The meter averages the whole frame. Works well when the scene is evenly lit and the composition isn't contrasty. Wedding mid-aisle shots, general street photography, flat-light portraits where the subject isn't fighting the background. Modern digital cameras default to matrix because it's right most of the time for most photographers. For film, it's fine for 80 percent of scenes.
Centre-weighted. Biases a larger circle in the middle of the frame, usually around 60 percent of the reading, with the edges contributing the remaining 40. This is classic 35mm camera metering. Better than matrix when your subject is roughly centred and you want the edges to matter but not dominate. stop. defaults to centre-weighted because it works for most street and portrait setups without needing the user to think.
Spot. Reads a small circle, about 2 percent of the frame. Point it at a specific tonal value and know the meter saw only that patch and nothing else. Indispensable for backlit subjects, high-contrast scenes, zone-system work, and anything where you are composing deliberately and want full control. Meter off a mid-grey, a white wall, or a face, and apply the mental offset for what tone you want it to fall on.
The shortcut: matrix when you are shooting fast and the light is forgiving, centre-weighted when you have time to compose, spot when the light is contrasty and you care about a specific tonal placement more than the average of the scene.
Some stocks forgive. Some punish a third of a stop.
Latitude is how far your exposure can be off before the image visibly breaks. Negative colour film, especially Portra, has enormous latitude. You can overexpose it by three full stops and still pull a clean scan. Slide film, Velvia and Provia, has almost none. Blow a highlight on Velvia by half a stop and the roll is ruined.
Knowing a stock's latitude tells you how carefully you need to meter for it. Meter tight for Velvia. Meter loose for Portra. The film tells you how precise to be.
| Stock | Type | Over | Under | Notes |
|---|---|---|---|---|
| Portra 400 | Colour neg | +3 stops | minus 1 stop | Loves overexposure. Rated 200 in practice. |
| Tri-X 400 | B&W neg | +2 stops | minus 2 stops | Forgiving both ways. Push-friendly. |
| Ektar 100 | Colour neg | +2 stops | minus half a stop | Narrower than Portra. Meter accurately. |
| Velvia 50 | Slide, E-6 | +1/3 stop | minus 1/3 stop | No room for error. Spot meter or walk away. |
| Cinestill 800T | Tungsten neg | +2 stops | minus 1 stop | Rate at 500 for clean shadows. Halation is the point. |
| HP5 Plus 400 | B&W neg | +2 stops | minus 2 stops | British weather's best friend. |
Meter tight for Velvia. Meter loose for Portra. The film tells you how precise to be.
When the film stops listening to the meter.
Film's response to light is linear for a narrow band of exposure times, roughly 1/1000s through 1 second. Outside that range, especially on long exposures, the emulsion stops behaving predictably. A metered 8-second exposure might actually need 20 seconds to reach the same density on the negative. This is reciprocity failure.
Every stock fails differently and every datasheet publishes its own correction table. Portra 400 needs roughly 1/3 stop extra at 1 second and a full stop at 10 seconds. Velvia 50 needs 1/3 stop at 1 second, 2 stops at 30 seconds, and becomes unreliable beyond a minute. Kodak and Fujifilm publish the numbers for every stock they make. Print them out and slip them in the back of your notebook if you shoot at night.
stop. applies reciprocity correction automatically for the six stocks in the Pro library. If you are shooting something not on the list, meter normally at whatever shutter speed you want, then add the correction from the datasheet. The meter isn't lying. The film just isn't a straight line any more.
Most of the time this will never come up. Street and portrait work happens in the 1/1000 to 1/60 range where reciprocity is effectively off. Reciprocity matters when you are shooting deep blue hour, astrophotography, or interior scenes at f/22 on a tripod. If you are doing any of those, this is the chapter that saves the roll.
Now go and meter the light.
You have the rule, the triangle, the three tricky scenes, the modes, the latitudes, and the one reason the film stops listening. Open the meter and calibrate.